Implantable Cardioverter Defibrillators (ICDs) are implanted in patients susceptible to cardiac tachyarrhythmias including atrial and ventricular tachycardias and atrial and ventricular fibrillation. Such devices typically provide cardioversion or defibrillation by delivering low voltage pacing pulses or high voltage shocks to the patient's heart, typically about 500-800V. The ICD operates by using sensors to detect a fast heart rate or tachyarrhythmia, upon which a battery within the device housing is coupled via an inverter to a high voltage capacitor or capacitor pair to charge the capacitors. When the capacitor reaches a desired voltage, charging is stopped and the capacitors are discharged under control of a microprocessor to provide a therapeutic shock to the patient's heart.
ICDs and other cardiac therapy devices such as pacemakers operate by sensing the cardiac rhythm or pulse rate of the patient in which a device is implanted, in order to detect abnormal rhythms requiring therapy for correction. Current devices provide the sensing function by way of a conductive probe extending from an implanted device to a chamber of the patient's heart, where a free end of the lead has a conductive portion that provides the device with information about the electrical characteristics (e.g. resistance) between the device and the probe tip. While effective, this sensing technique may be substituted or supplemented by the use of photoplethysmography.
Photoplethysmography exploits the nature of some tissue to change its optical characteristics based on cardiac rhythm, with a detectable repeating pattern that follows the pulse. Unlike conventional pulse oximetry, which detects changes in light transmission through tissue such as a fingertip or earlobe based on the characteristics of the blood within the tissue (e.g. oxygen level), photoplethysmography detects blood flow characteristics based on the changing reflective characteristics of the surface of tissue. It has been observed that the vasculature of tissue has changing reflective characteristics of certain wavelengths of light during the cardiac rhythm cycle. (The term “light” is intended to include non-visible electromagnetic radiation such as infra-red.) Specifically, a pulse of decreased reflectivity of red or near infra-red light is observed to correlate with a patent's patient's systolic pulse. This is most readily observed with internal tissue where the vasculature is at the surface, as opposed to external tissue where the vasculature is well below the surface.
The use of photoplethysmography has been found to have limited applications for use with implanted devices. A primary concern is that the technology requires an optically-transmissive “window,” while most implanted device circuitry requires a metal housing to prevent external electronic interference from interfering with device operations. The transparent materials available for this window lack the shielding characteristics required to prevent potentially harmful interference. Contemplated alternatives for implantation of separate interconnected devices (one with a metal housing, the other transparent) may create disadvantageous complexity of connections and surgical procedures.
Other prior art devices employ emitters and detectors outside the device housing, but encapsulated by a transparent epoxy material that forms the device header. These suffer the disadvantage that the emitter and detector components are vulnerable to intrusion of body fluids, because an epoxy encapsulation is inadequate to provide a seal against such incursion. Accordingly, such components must be specially selected or manufactured to ensure that they are formed of biocompatible materials that are not degraded by body fluids, and which do not generate harmful materials as a result of such fluid contact that may leach back to body tissues and cause harm. This concern includes exposed lead wires, any required insulation, and materials used for soldering or welding. Conventional cost-effective LED lamps are further believed not to be hermetic, so that internal components must also be selected for biocompatibility, and incur the risk that degradation or failure may occur upon incursion of body fluids.
Devices that include integrated sensors, even if outside of the main hermetically sealed chamber, and with hermetically sealed sensors may have limitations in certain respects. With the complexity of development and assembly of both the sensors and the ICD itself, a fully integrated design can present challenges. Due to ongoing efforts to miniaturize and otherwise improve both ICD circuitry and sensor elements, development of one aspect can affect compatibility with another aspect, necessitating redesigns of the other component. Moreover, with the added complexity of the overall system, there may be an increased yield loss during manufacturing as, for instance, a faulty sensor that is integrated with an ICD may necessitate repair or loss of the entire device.